1. Field of Invention
The present invention relates to a method and system for controlling a downshift for a hybrid vehicle. More particularly, the present invention relates to a method and system for controlling a downshift for a hybrid vehicle that performs an operation of a downshift along with an operation of engagement of an engine clutch installed between an engine and a motor when a downshift is required in response to a kick-down while a hybrid vehicle runs in an electric vehicle (EV) mode, thereby reducing a feeling of acceleration delay and improving acceleration linearity.
2. Description of Related Art
Hybrid electric vehicles operate using power from an internal combustion engine and power from a battery. In particular, hybrid vehicles are designed to efficiently combine and use power of the internal combustion engine and the motor.
For example, as illustrated in FIG. 1, a hybrid vehicle includes an engine 10, a motor 20, an engine clutch 30, a transmission 40, a differential gear unit 50, a battery 60, an integrated starter-generator (ISG) 70, and wheels 80. The engine clutch 30 controls power transmission between the engine 10 and the motor 20, and the integrated starter-generator (ISG) 70 starts the engine 10 or generates electric power by an output torque of the engine 10.
As shown in FIG. 1, in case the engine clutch 30 is installed between the engine 10 and the motor 20, since the motor 20 is typically mounted on a transmission, the hybrid vehicle shown in FIG. 1 is referred to as a transmission electric device (TMED) type of hybrid vehicle.
As also shown in FIG. 1, the hybrid vehicle further includes: a hybrid control unit (HCU) 200 which controls overall operation of the hybrid electric vehicle; an engine control unit (ECU) 110 which controls operation of the engine 10; a motor control unit (MCU) 120 which controls operation of the motor 20; a transmission control unit (TCU) 140 which controls operation of the transmission 40; and a battery control unit (BCU) 160 which manages and controls the battery 60.
The battery control unit 160 may also be referred to as a battery management system (BMS). The integrated starter-generator 70 may also be referred to as a starting/generating motor or a hybrid starter-generator.
The hybrid vehicle may run in a driving mode such as an electric vehicle (EV) mode using only power of the motor 20, a hybrid electric vehicle (HEV) mode using torque of the engine 10 as the main power and torque of the motor 20 as auxiliary power, and a regenerative braking (RB) mode during braking or when the vehicle runs by inertia. In the RB mode, braking and inertia energy are collected through power generation of the motor 20, and the battery 60 is charged with the collected energy.
In case the hybrid vehicle is a TMED type in which an automatic transmission and/or a DCT are installed, when a driver's demand torque suddenly increases (for example, a kick-down occurs) while the hybrid vehicle runs in an electric vehicle (EV) mode, downshift control may be performed. The downshift control is control that changes the current gear shift stage to a lower gear shift stage.
Engagement of the engine clutch is required to perform the downshift control.
While the downshift control (or operation) is performed, engaging the engine clutch along with shifting is preferred. In the related art, however, because of complexity of shifting control and controlling the engine clutch, a shift operation is performed after engaging the engine clutch or an engagement operation of the engine clutch is performed after shifting, thus it is difficult to satisfy a driver's acceleration demand and/or demand torque.
FIG. 2 is a graph for showing a method of performing shift operation after engaging an engine clutch, and FIG. 3 is a graph for showing a method of performing an engagement operation of an engine clutch after shifting.
As shown in FIGS. 2 and 3, when shifting control and engagement control of the engine clutch are independently performed, it may be easy to improve drivability since problems related to the shifting control and the engagement control of the engine clutch are not overlapped. However, in case of performing the shifting control and the engagement control of the engine clutch independently, acceleration performance may deteriorate since it takes a considerable time to satisfy a driver's acceleration demand and/or demand torque.
As shown in FIG. 2, the method of performing the shift operation after engaging the engine clutch may obtain linear acceleration performance after shifting, but a torque applied to a wheel may be increased slowly because of performing the shift operation after engaging the engine clutch.
As shown in FIG. 3, since the method of performing the engagement operation of the engine clutch after shifting first performs the shifting, a torque applied to a wheel may be rapidly increased. However, since the torque applied to the wheel is sufficiently generated after the engagement of the engine clutch, a difference between torques applied to the wheel occurs, thus acceleration linearity may deteriorate.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.